The invention relates to a method for diagnosing an exhaust gas cleaning system in the exhaust section of a lambda-controlled internal combustion engine having a 3-way catalytic converter, a binary pre-catalyst lambda sensor connected upstream of the catalytic converter, and a post-catalyst lambda sensor connected downstream of the catalytic converter.
In an exhaust gas cleaning system with two lambda sensors, a pre-catalyst lambda sensor upstream of the catalytic converter is used as a measuring sensor. A post-catalyst lambda sensor downstream of the catalytic converter serves as a monitoring sensor for monitoring and compensating for a change in the static or dynamic properties of the pre-catalyst lambda sensor which would lead to an increase in emissions. Usually, both lambda sensors have a two-point characteristic, and the voltage signal which they emit is dependent on the oxygen content in the exhaust gas. The oxygen content in the exhaust gas is in turn dependent on the mix which has been fed to the internal combustion engine. If the mix is lean (lambda greater than 1), the output voltage from a binary lambda sensor is usually below 100 mV, changes almost as an abrupt jump in the region of lambda=1 and, when the mix is rich (lambda less than 1), reaches over 0.7 V; this is known as a two-point characteristic.
The dynamic and static properties of the pre-catalyst lambda sensor are altered by sensor aging and poisoning. As a result, the control position of the lambda control is shifted. For example, phosphorus poisoning may lead to an asymmetrical change in the sensor response time and therefore to a lean shift in the sensor control out of the optimum lambda range for the catalytic conversion. As a result, by way of example, the NOx emissions may rise beyond a permitted limit. The post-catalyst lambda sensor is used as a monitoring sensor for monitoring the catalytic conversion and for fine control of the mix in order always to be able to maintain the lambda value which is most favorable for conversion. Use is made of the fact that the two-point characteristic of the post-catalyst lambda sensor, on account of the conversion capacity, which also has a damping effect, of the catalytic converter, approximately changes into a linear characteristic within a very limited lambda range. This method, which is usually known as trimming or guide control, is known, for example, from DE 35 00 594 C2.
The prior art has disclosed numerous methods for diagnosing or checking a catalytic converter, for example DE 41 28 823 A1, which determines the oxygen storage capacity of a 3-way catalytic converter by recording the time required to empty or fill the catalytic converter with oxygen and calculating the quantity of oxygen from this time. For this method, a lambda sensor with wide-band characteristics, i.e. the signal of which changes proportionally to the lambda value, is imperative upstream of the catalytic converter.
However, the known methods have the drawback that, if the pre-catalyst lambda sensor is defective, a defective catalytic converter is often diagnosed, even though unacceptable levels of emissions are not yet being discharged. According to the prior art, it is not possible to differentiate between a catalytic converter defect and a failure of the pre-catalyst lambda sensor. Furthermore, the high oxygen loading/discharge, which is required for diagnosis according to the prior art, often leads to an undesirable level of pollutant emissions, since the catalytic converter is then no longer operated in its optimum range. Finally, wide-band lambda sensors are relatively expensive. Methods which are based on less expensive binary lambda sensors, i.e. lambda sensors with a two-point characteristic, have not hitherto been worthy of comparison with regard to their diagnosis capacity.
It is an object of the present invention to provide a method for diagnosing an exhaust gas cleaning system of a lambda-controlled internal combustion engine which does not necessarily require the use of a binary lambda sensor and which makes it possible to differentiate between a defect in the pre-catalyst lambda sensor and a failure of the catalytic converter.
This object is achieved by the invention wherein the oxygen storage capacity of the catalytic converter, which is a measure of its conversion capacity, is used to diagnose the exhaust gas cleaning system. The catalytic converter is subjected to a load which is brought about by fluctuation of the fuel/air ratio about the stoichiometric point and which is greater than the operating load which occurs with standard binary lambda control. However, a certain maximum catalytic converter load is not exceeded. The load is predetermined by influencing the oscillation of the fuel/air ratio and therefore the signal from the pre-catalyst lambda sensor. For diagnosis, the oscillation characteristic of the signal from the post-catalyst lambda sensor is evaluated. The fluctuating load is adsorbed to a greater or lesser extent in the catalytic converter depending on the oxygen storage capacity of the catalytic converter. The oscillation of the signal from the post-catalyst lambda sensor therefore differs depending on the oxygen storage capacity of the catalytic converter. Measurements carried out on a catalytic converter which has just fallen below the predetermined oxygen storage capacity, and therefore the desired conversion capacities, can be used to determine a desired range for the oscillation characteristic of the signal from the post-catalyst lambda sensor. A catalytic converter of this type is usually known as a limit catalytic converter.
If the oscillation is greater, i.e. the amplitude or included area is larger, the diagnosed catalytic converter is worse than the limit catalytic converter.
In a preferred embodiment of the method, the loading of the catalytic converter is set by controlling the fluctuation of the fuel/air ratio in such a way that the oscillation of the signal from the pre-catalyst lambda sensor includes a certain minimum area. The oxygen mass mO2 which is introduced into the catalytic converter within a half period of the oscillation can be calculated according to the following equation:
mO2=21%(lambdaxe2x88x921)/lambda MAF Txc2xd
where MAF is the mass flow of fresh gas sucked in. mO2 is the oxygen mass which is fed to the catalytic converter during the half period of the lambda control in which the mix is lean. By varying the duration of the half period Txc2xd, it is therefore possible to set the catalytic converter loading, i.e. the quantity of oxygen introduced into the catalytic converter. By correspondingly varying the duration of the other (rich) half period, it is possible to vary the catalytic converter load and nevertheless to keep the mix stoichiometric on average. The terms xe2x80x9cloadingxe2x80x9d and xe2x80x9cloadxe2x80x9d are therefore used interchangeably.
In a preferred embodiment of the invention, with a binary lambda control the P-jump delay time is adjusted, with the result that Txc2xd changes. To ensure that the internal combustion engine is nevertheless on average supplied with a stoichiometric mix, the P-jump delay time must vary in the same way both for the jump from lean to rich and for the jump from rich to lean. The catalytic converter load which is predetermined in this manner can be oriented to the emission limits which are to be checked.
Stipulating the loading has the advantage of being significantly less sensitive to load/rotational speed variations than known methods. Particularly at low engine loads or rotational speeds, good diagnosis of the catalytic converter is still possible.
The trimming control likewise has an effect on the P-jump delay time, but only on the delay time for one jump, either from lean to rich or from rich to lean. This different or one-sided change in the P-jump delay time means that the trimming control compensates for age-related changes in the pre-catalyst lambda sensor. Changes of this type generally lead to a shift in the lambda=1 working point of the sensor.
If it is established that the durations of one or both half periods cannot be increased as desired, since the acceptable P-jump delay time would be exceeded either for the lean to rich jump or for the rich to lean jump, and the diagnosis through the monitoring of the signal from the post-catalyst lambda sensor nevertheless indicates a defective exhaust gas cleaning system on account of an unacceptable oscillation in the signal from the post-catalyst lambda sensor, the pre-catalyst lambda sensor is defective. In such a case, the trimming control has compensated for a fault in the pre-catalyst lambda sensor all the way to the maximum permissible degree. Therefore, in all probability the unacceptable oscillation in the signal from the post-catalyst lambda sensor during diagnosis of the exhaust gas cleaning system is attributable to a defective pre-catalyst lambda sensor.
The signal from the post-catalyst lambda sensor is preferably evaluated by determining the area included by the oscillation of this signal. The period which is predetermined by the fluctuation in the fuel/air ratio and therefore by the oscillation in the signal from the pre-catalyst lambda sensor is of crucial importance to the desired characteristic of the oscillation. Therefore, the area which is included by the oscillation of the signal from the post-catalyst lambda sensor can be determined particularly easily by integrating or summing the amount of deviation from the mean of the signal from the post-catalyst lambda sensor over one period of the oscillation of the signal from the pre-catalyst lambda sensor. This period-synchronous integration eliminates the period determination at the signal from the post-catalyst lambda sensor.
A value which is characteristic of the oxygen storage capacity of the catalytic converter can be obtained from this integral or sum which describes the area. It is possible to weight this value still further with operating parameters such as rotational speed and engine load to give an emission index, so that the diagnosis can be made independently of an emission limit value being exceeded.
Furthermore, the diagnosis will preferably be discontinued when the lambda control presents a certain control deviation. In such a case, a load caused by varying the P-jump delay time, for example, would lead to an unacceptable increase in emissions.
Advantageous refinements of the invention form the subject matter of the subclaims.